How to certify a ring main with lighting circuit taken of a fused spur???

[newbie111]

just wondering how i would certify this type of installation, would i take the highest resistance value of r1+r2 of the lighting circuit and use this to add to ze? would i also have seperate values for the two circuits recorded on the EIC. any help eould be very welcome

 

[marcusone]

Hi Newbie

I would record the lighting circuit separately on the EIC but make it clear that it is a spur off the ring main. The lighting circuit has its own protective device and Ze for the lighting circuit will be measured at the fused spur.

Cheers

Mark.

 

[newbie111]

thanks for your prompt reply, so the fuse will be the protective device for the lighting spur and not the mcb at the distribution board? so the lighting circuit is tested as a completely seperate circuit and recorded assuch. thanks again
also if the ring trips and the lights also go out.As they are feed from the ring,does it comply to 17th as we must minimise inconvinence to users?

 

[malcolmsanford]

Well you answered your own question, it's not a great design to take a lighting circuit of a ring final circuit, which is why you don't often see it. There is nothing in the regs to stop you doing it, as whoever did it thought that was the best design for the circumstances, and who can say he/she was wrong, as we don't know what the circumstances were.

 

[newbie111]

where would i find the zs for a 3amp fuse,if that is the protective device and would i do pfc and pefc at that point too,thanks again.

 

[chiplard]

Hi Newbie

I would record the lighting circuit separately on the EIC but make it clear that it is a spur off the ring main. The lighting circuit has its own protective device and Ze for the lighting circuit will be measured at the fused spur.

Cheers

Mark.

The Ze is ONLY measured at the Supply. A Zs figure should be measured at the furthest point of the lighting circuit to ensure it meets the tabulated values for the MCB.

 

[newbie111]

so the circuit would be measured for zs at the furthest lighting point and should not be done seperately for the ring final and lighting circuit,sorry getting a bit confused on how i should complete an EIC with this circuit?

 

[DurhamSparky]

do the test as usuall!!

include the spur as part of the ring! as effectively thats what it is..! go to th furthest light and get a zs..! if your ZS is Higher that the ring ZS note that one down! but make sure the cert is naoted that its a socket / light circuit!

 

[newbie111]

thank you

 

[DurhamSparky]

welcome

 

[gamblor6]

agreed durham. its never its own circuit if its spurred off a ring.

 

[gamblor6]

ze from the fcu ive heard it all now lol

 

[DurhamSparky]

creating a new circuit with in a circuit on its own protective device as well!! ermmmm!! wtf! surely at some point ALL the circuits will be on a 32 unless its a magic FCU that stays on when the MCB trips!

 

[Deleted member 19807]

I have started to record spurs with an "a" after the circuit number with the details of the fuse in the fcu as the over current device, so if the ring is on circuit 3, the spur will be 3a, I just think that anyone looking at it later when trying to sort out a problem will then know there is an fcu controling " outside lights" as an example.

May be a bit ott but not a lot of extra work.

 

[newbie111]

just trying to get this correct( which for my small brain is something), zs is taken from the furthest lighting point and is the zs for the ring final and lights combined. The 5amp fused spur which feeds the lights will have a zs value for the fuse disconection time and this will be recorded on a seperate line and noted as a spur.
The recorded value(R1+R2 )for the lights is 1.27 ohms at the furthest point,but this is to high for a type b 32 A circuit breaker i believe. So the lighting circuit needs to be re-routed or cable csa increased?
Also the installation is a TNS, the main earth connection to the MET is 10 mm,but then has been increased to 16 mm from the met to the consumer unit. Is that acceptable? thanks in advance

 

[DurhamSparky]

Record the Highest ZS you get from the circuit, IGNORE the 5amp FCU that only protects the cable and not used for disconnection times just over load on the cable

Refer to the 32A MCB that is in the CU for Disconnection times which i believe is 1.15 ohms unless its protected by a RCD then it can be up to 1667 ohms.

so your 1.27 ohms is WELL within limits for disconnection by a RCD

TNS system should be 16mm earth to the CU and the bonding in 10mm

 

[newbie111]

thanks steve, thats a great help to me. I did not realise the rcd increased the value up to 1667, thought that was just for TT system. Im learning a lot from you guy's so thanks again for all your help.

 

[newbie111]

one last question, lf using a wander lead connected to the earthing terminal firstly and then the line conductor(at the consumer unit) and measuring from those points to the line and cpc at the furthest lighting point and the corresponding terminals. will ,when added together with ze will equal zs?
zs=ze(R1+R2) would this be one way to calulate zs for the lighting circuit?

 

[malcolmsanford]

That is a novel way to get your R1+R2 using a wander lead, normally you only use the wander lead for R2. To save you trailing a lead around just link the LINE/CPC at the CU and then take your readings at the points to get R1+R2.

Yes quite right by getting your R1+R2 and then adding the Ze will give you your Zs. This is often the way it's done on initial verifications. For a PIR I have to admit I would do a live test espicially in a commercial/industrial enviroment. I do realise that by doing it live you can introduce parallel paths, but IMO that is not a bad thing really, as that will be the status of the installation if a fault occurs.

Now with RCD protection so prevelant and "soft" test being done to avoid tripping, you can produce very high results on a Zs. Though the flip side to this is becasue of the use of RCDs as additional protection the Zs reading is becoming less and less critical as 1667 is your important figure and if you can't achieve below this then you have serious problems.

 

[newbie111]

thanks malcom. For this installation which has a lighting circuit connected through a fused spur,should i do the figure of 8 for the ring and then test the furthest lighting point with line/cpc crossed connected at the furthest lighting point and that will give me R1+R2?
, thanks again .